Semiconductor lasers (or in short laser diodes) have become part of our daily life. They are robust, price competitive and capable of converting electric power into light with up to 70% efficiency. Some of the most well-known consumer applications are DVD players and laser pointers, where the requirements regarding output power are not so demanding. For professional applications, the power level needed is significantly higher, and requires the implementation of dedicated high power designs. However, increasing the power generally comes at a cost in terms of beam quality.
Historically, laser diodes were used to pump solid state lasers and employed in material processing, like heat-treating of metal for hardening, which required no particularly good beam quality and consequently no high brightness. Nevertheless high-power laser diodes, in particular single emitters and bars are developing fast. The achievable power per device is rising, the efficiency is increasing, and the facet damage threshold is continuously improving. Nevertheless, their poor beam quality has prevented laser diodes from being successfully used for welding, cutting and marking, applications, which are dominated by lamp and diode pumped solid-state lasers that exhibit about an order of magnitude higher brightness than semiconductor lasers. Over the last years, new semiconductor laser diode designs, capable of making high-power diode beams brighter, have been developed. Not surprisingly, therefore, direct diode systems are becoming more and more competitive in applications requiring good beam quality and high power. The industrial interest is related to the potential enormous increase in efficiency from current 2 to 30% for optically pumped laser systems to expected over 40 to 60% for direct diode laser systems. High efficiency speaks for simplified cooling systems, less electrical power consumption and reduced operating costs. Additionally, laser diode systems show excellent reliability, providing multiple years of maintenance-free operation. The current trend suggests that diode lasers will replace optically pumped lasers in the near future, first in the low power and later also in the higher power classes. Beside the replacement of existing technology, high brightness laser diodes open up new applications like in laser projection and display systems for large screen, as well as in home entertainment systems. The advances in laser-diode technology that have enabled direct diode applications have been driven by increases in the power-per-laser-diode emitter, reductions in the beam divergence and improvements that have been made to collimating and beam-combining optics. However, the edge-emitting laser diode facet technology has, by far, had the greatest impact on the overall approach. Now, developers are pushing to improve beam quality in order to be able to put the high efficiency of diodes to work in high-power applications, such as, e.g. cutting and welding of metal sheets.
Within the numerous new laser designs aiming at the improvement of laser brightness, tapered laser diode technology is especially promising. Tapered laser diodes or tapered amplifier diodes can provide exceptionally bright, high-quality output with powers up to 10 W. The close to single mode beam quality enables highly effective fiber coupling into small fibers and brings within reach even direct nonlinear harmonic generation. Moreover, by employing newly developed beam combining schemes, the power could be scaled up while preserving a good beam profile. The high brilliance and power make real industrial deep-penetration welding possible with a direct-diode laser, achieving results comparable to optically pumped solid-state lasers.